1. Field of the Invention
The present invention relates generally to gyroscopic reference apparatus, such as an inertial reference unit for an aircraft inertial navigation system (INS) or a strapped-down attitude and heading reference system (SAHRS) for an aircraft, hereinafter simply referred to as an inertial measurement unit or IMU. More particularly, the invention relates to an adjustable mounting tray for supporting the IMU in a precisely aligned position relative to the aircraft's primary coordinate system, i.e., its vertical, lateral and longitudinal axes.
2. Description of the Prior Art
A typical strapped-down attitude and heading reference system is disclosed in the present assignee's U.S. Pat. No. 4,212,443.
The mounting of an IMU, or equivalent element of an integrated SAHRS or INS, in an aircraft under construction presents no particular problem because the shelf for supporting the IMU may be readily precisely aligned with the aircraft's primary axes using the precision manufacturing jigs. However, the retrofit installation of an IMU in an aircraft does present a difficult problem since the shelf dedicated to the support of the IMU may not be precisely aligned with the aircraft axes or even a new shelf specifically installed for the IMU may not and generally will not be aligned to the required degree of accuracy; for example, on the order of .+-.0.2 degrees. While retrofit alignment relative to the vertical and lateral axis is relatively simple using precision levels, such as liquid levels, precise alignment with the longitudinal axes is very difficult because of the inaccessability to this physical axes in the aircraft.
Presently known techniques used heretofore involved leveling the aircraft in pitch and roll in accordance with the aircraft manufacturer's specific instructions, aligning the IMU mounting tray with the aircraft principal axes to the required degree of accuracy using elaborate and expensive optical alignment jigs and tools, temporarily clamping the tray in place and then match-drilling installation holes and finally bolting the tray in place. This technique is awkward and time-consuming and the potential for installation errors is high, with the correction for such errors being quite difficult and expensive.